1. Field of the Invention
The invention relates to cassettes for magnetic tape used for recording and playback of information; and more particularly, to such cassettes having guides for maintaining accurate angular alignment of tape passing an open front region through which a magnetic head passes to contact to the tape, and to a method of manufacturing such a cassette.
Cassettes of this general type are used on a large scale in apparatus for many uses, both in the consumer goods sector and for professional and business activity. A particular style of cassette with which the invention is especially useful is commonly known as the "compact cassette," "standard cassette" or "Philips cassette;" external dimensions and preformance characteristics of these cassettes are standardized through adherence to International Electrotechnical Commission publication no. 94A. This cassette has established "support planes" defined by locating surfaces, from which critical dimensions for tape alignment are measured. Therefore, when designing high quality cassette apparatus, such as a high-fidelity audio recording and/or playback machine, customarily referred to as a tape recorder, for optimum results the recording/playback head is located accurately with respect to mounting surfaces which engage these locating surfaces.
When relatively high frequencies are to be handled by a tape recorder, it is critical that the tape runs exactly perpendicular to the gap in the magnetic head. Deviation from perfect perpendicularity produces what is called, in professional jargon, azimuth error. In the case of the compact cassette, this error is observed as the result of misalignment between tape guides located inside the cassette, over which tape is stretched as it passes the openings of the front wall region, and locating surfaces on the lower exterior of the cassette housing. Of course, when the cassette is turned over to play or record the other tracks on the same run of tape, the locating surfaces for what had been the top of the cassette now become the reference points for establishing the support planes.
Because of the vast quantities of compact cassettes which are made and sold, economy of construction is maximized wherever possible. This goal has dictated, for example, the use of relatively inexpensive low rigidity plastic materials for the cassette housing, which materials can usually be molded to produce interlocking shapes. As a result of this material choice, and the preference for simplest possible molds, azimuth error is an important source of limitation on the high frequency response of the cassette.
2. Description of the Prior Art
The original compact cassette is described in U.S. Pat. No. 3,394,899 and uses a housing made of two identical halves, each including one pair of locating surfaces and half of the guide surfaces. Each half is box-shaped and has a bottom with openings for the passage of reel drive spindles, cassette positioning pins, and a drive capstan. Each housing half has half-height side and rear walls. Along the front edge or wall a number of openings are produced, for the heads and pressure rollers to pass through.
To guide a tape passing by the front openings, a tape guide unit must be provided. In the conventional two-part cassette housing, half-height guides project substantially transversely to the bottom (and the cover, which is simply "the bottom" of another half turned upside down). However, when molding such a housing half it is necessary to provide a taper or "draft" in those parts which extends substantially in the direction in which the mold is opened, so that the housings can be removed easily. Because of this draft the tape guides are not true cylindrical surfaces about axes perpendicular to the support planes of the cassette, but have more of a conical shape. Azimuth errors of plus or minus three minutes of arc are inevitable with this style of construction.
In contradistinction to the conventional construction described above, U.S. Pat. No. 3,934,842 teaches a three-part housing, also made of moldable plastic parts. In this cassette the tape guide unit is a separate molding which can be made from a plastic having desirable low-friction properties. Further the tape guide unit is molded with the mold interface perpendicular to the bottom cassette wall, so that the sliding surfaces provided for tape guiding can be theoretically perpendicular to the support planes. This separate guide unit is then mounted in the box-shaped cassette part. However, the accuracy depends on the cumulative dimensional tolerances of the box-shaped outer part and the tape guide unit, and also on the tolerances observed in assembly per se. Moreover, the top and the bottom of the guide unit each are tapered to provide draft, so that these surfaces by which the tape guide unit is mounted in the cassette may again provide an oblique relationship of the tape guides relative to the bottom of the box-shaped part. This construction also has the disadvantage that rigidity of the cassette housing is necessarily reduced by making the assembly from three parts.
A further feature of the cassette described in U.S. Pat. No. 3,934,842 is that all the surfaces in the region projecting forward from the tape guide depend from the cover. This prevents use of identical upper and lower halves, but enables easy threading of the tape onto the lower housing when the cassette is being assembled, the cover being attached last and partially enclosing the front.
Still another construction and related manufacturing method for cassettes is known in the field of so-called professional or digital cassettes. These cassettes utilize a metal cassette frame, which constitutes the tape guide unit and the side walls of the cassette housing; and two thin plastic covers which are mounted on the upper and lower sides of the metal frame. Such cassettes have been available since approximately 1971; for example, under the designation Philips model LGH 6003 or LGH 6005. The metal frame of this three-part cassette is cast in a composite mold having a movable third mold section which can form cassette guiding surfaces which are true cylinders perpendicular to the support planes. As a result of this high quality and expensive construction these professional cassettes provide correct tape guidance with only a very small tolerance on the azimuth angle, and are quite rigid. This rigidity is obtained almost entirely from the rigidity of the metal frame, the covers being comparatively thin and readily deformable. These professional cassettes are not readily suited for use in consumer equipment, however, because there is no provision for the break-away tabs in the rear wall (opposite to the front wall opening through which heads enter) which are used to code the type of tape used in the cassette, or whether or not the cassette is pre-recorded. Many consumer-type cassette apparatus contain sensors for detecting the presence or absence of such tabs, so as to inhibit erasing of pre-recorded tape, or to change the bias current used for recording to match most closely the properties of the magnetic tape with which the cassette has been loaded. Further, such code openings or tabs cannot be readily provided in the rear wall of a cassette housing utilizing the professional cassette style metal frame. Because the frame is essentially a hollow ring, the provision of such an opening would lead to an unacceptable localized reduction in strength, and might even seriously impair its structural integrity.